At this point you should have figured
outthat without air masses there
would be nofronts. The centers of
action are responsiblefor bringing
the air masses together and formingfrontal
zones.

The primary frontal zones of the Northern Hemisphere
are the arctic frontal zone andthe
polar frontal zone. The most importantfrontal
zone affecting the United States isthe
polar front. The polar front is the regionof
transition between the cold polar air andwarm
tropical air. During the winter months(in
the Northern Hemisphere), the polar frontpushes
farther southward, because of the greaterdensity
of the polar air, than during the summermonths.
During the summer months (in theNorthern
Hemisphere), the polar front seldommoves
farther south than the central UnitedStates.

On a surface map a front is indicated by a line
separating two air masses; this is only apicture
of the surface conditions. These airmasses
and fronts extend vertically. (See fig.4-2-1.)

A cold air mass, being heavier, acts like a
wedge and tends to underrun a warm airmass.
Thus, the cold air is below and thewarm
air is above the surface of discontinuity.This
wedge of cold air produces a slope ofthe
frontal surface. This slope is usually be-tween1
to 50 (1 mile vertical for 50 mileshorizontal)
for a cold front and 1 to 300(1
mile vertical for 300 miles horizontal) fora
warm front. For example, 100 miles fromthe
place where the frontal surface meets theground,
the frontal surface might be some-wherebetween
2,000 feet and 10,000 feet aboveEarth’s
surface, depending on the slope. Theslope
of a front is of considerable impor-tancein
visualizing and understanding the weatheralong
the front.

RELATION OF FRONTS
TO
CYCLONES

There is a systemic relationship between cyclones
and fronts, in that the cyclones are usually
associated with waves along fronts— primarily cold fronts. Cyclones come into
being
or intensify because pressure
falls more rapidlyat one point
than it does in the surrounding area.Cyclogenesis
can occur anywhere, but in middleand
high latitudes, it is most likely to occur on

Figure
4-2-1.—Vertical view of a frontal system (clouds not shown).

a
frontal trough. When a cyclone (or simply low)develops
on a front, the cyclogenesis begins at thesurface
and develops gradually upward as thecyclone
deepens. The reverse also occurs; closedcirculations
aloft sometime work their waydownward
until they appear on the surface chart.These
cyclones rarely contain fronts and are quasi-stationaryor
drift slowly westward and/orequatorward.

Every front, however, is associated with a

cyclone.
Fronts move with the counterclockwiseflow
associated with Northern Hemispherecyclones
and clockwise with the flow of SouthernHemisphere
cyclones. The middle latitudes areregions
where cold and warm air masses con-tinuallyinteract
with each other. This interactioncoincides
with the location of the polar front.When
the polar front moves southward, it isusually
associated with the development andmovement
of cyclones and with outbreaks of coldpolar
air. The cyclonic circulation associated withthe
polar front tends to bring polar air southwardand
warm moist tropical air northward.During
the winter months, the warm airflowusually
occurs over water and the cold air movessouthward
over continental areas. In summer thesituation
is reversed. Large cyclones that form onthe
polar front are usually followed by smallercyclones
and are referred to as families. Thesesmaller
cyclones tend to carry the front farthersouthward.
In an ideal situation these cyclonescome
in succession, causing the front (in theNorthern
Hemisphere) to lie in a southwest tonortheast
direction.

Every moving cyclone usually has two signifi-cant lines
of convergence distinguished by ther-malproperties.
The discontinuity line on theforward
side of the cyclone where warm airreplaces
cold air is the warm front; the discon-tinuityline
in the rear portion of the cyclone wherecold
air displaces warm air is the cold front.The
polar front is subject to cyclonic develop-mentalong
it. When wind, temperature, pressure,and
upper level influences are right, waves formalong
the polar front. Wave cyclones normallyprogress
along the polar front with an eastwardcomponent
at an average rate of 25 to 30 knots,although
50 knots is not impossible, especially inthe
case of stable waves. These waves mayultimately
develop into full-blown low-pressuresystems
with gale force winds. The developmentof
a significant cyclone along the polar frontdepends
on whether the initial wave is stable orunstable.
Wave formation is more likely tooccur
on slowly moving or stationary frontslike
the polar front than on rapidly movingfronts.
Certain areas are preferred localitiesfor
wave cyclogenesis. The Rockies, the Ozarks,and
the Appalachians are examples in NorthAmerica.

Stable Waves

A stable wave is one that neither develops nor

occludes,
but appears to remain in about the samestate.
Stable waves usually have small amplitude,weak
low centers, and a fairly regular rate anddirection
of movement. The development of astable
wave is shown in views A, B, and C offigure
4-2-2. Stable waves do not go into a growthand
occlusion stage.

Unstable Waves

The unstable wave is by far the more common

wave
that is experienced with development alongthe
polar front. The amplitude of this waveincreases
with time until the occlusion processoccurs.
The formation of a deep cyclone and anoccluded
front breaks up the polar front. Whenthe
occlusion process is complete, the polar frontis
reestablished. This process is shown in figure4-2-3.
Views A through G of figure 4-2-3, refer-redto
in the next three paragraphs, show the lifecycle
of the unstable wave.

In its initial stage of development, the polar

front
separates the polar easterlies from the mid-latitudewesterlies
(view A); the small disturbancecaused
by the steady state of the wind is often notobvious
on the weather map. Uneven localheating,
irregular terrain, or wind shear betweenthe
opposing air currents may start a wavelike per-turbationon
the front (view B); if this tendencypersists
and the wave increases in amplitude, acounterclockwise
(cyclonic) circulation is set up.One
section of the front begins to move as a warmfront
while the adjacent sections begin to moveas
a cold front (view C). This deformation is calleda
frontal wave.

The pressure at the peak of the frontal wave

falls,
and a low-pressure center is formed. Thecyclonic
circulation becomes stronger; the windcomponents
are now strong enough to move thefronts;
the westerlies turn to southwest winds andpush
the eastern part of the front northward asa
warm front; and the easterlies on the westernside
turn to northerly winds and push the westernpart
southward as a cold front. The cold frontis
moving faster than the warm front (view D).When
the cold front overtakes the warm front andcloses
the warm sector, an occlusion is formed(view
E). This is the time of maximum intensityof
the wave cyclone.

As the occlusion continues to extend outward,

the
cyclonic circulation diminishes in intensity (thelow-pressure
area weakens), and the frontal move-mentslows
down (view F). Sometimes a newfrontal
wave may begin to form on the westwardtrailing
portion of the cold front. In the finalstage,
the two fronts become a single stationaryfront
again. The low center with its remnant ofthe
occlusion has disappeared (view G).

Table 4-2-1 shows the numerical average life

cycle
of a typical unstable wave cyclone frominitial
development to cyclolysis. It is onlyintended
to be used as a guide in areas wherereports
are sparse.

Learning Objective: Describe the condi-tions
necessary
for frontogenesis and